# Concept of Entropy. (Question)

## Main Question or Discussion Point

I have a question about the increasing behavior of entropy.
Suppose we heat a metal (Take for instance Fe) till it radiates energy and then put it in space (No medium). So the metal radiates energy in electromagnetic waves which decreases the entropy of the metal (Due to decrease in internal energy of metal. I hope so ).On the other hand the E.M. waves will keep traveling in space (no medium.It might get absorbed by other objects but still space is quite large?)(The space will be the dump for this energy).So the net entropy change will be a decrease (due to decrease in the system (Fe metal)).But second law of thermodynamics states that entropy always increases for irreversible change.

Where have I gone wrong?

Help will be appreciated.Thank you .

Related Classical Physics News on Phys.org
Drakkith
Staff Emeritus
Have you considered the source of your energy you used to heat the metal?

mfb
Mentor
The closed system here is your whole space - and radiation has entropy, too.

Have you considered the source of your energy you used to heat the metal?
Ok . If i heat the metal by keeping it in contact with body at high temperature T2 then it gives heat q to the metal , So entropy of the body at T2 decreases, right??

The closed system here is your whole space - and radiation has entropy, too.
Could you explain, please? I do not have deep knowledge in thermodynamics.

mfb
Mentor
But second law of thermodynamics states that entropy always increases for irreversible change.
This is true for closed systems only. If you have some way to let entropy flow out, entropy of an object can decrease. This is done all the time when objects cool: They transfer entropy to the environment around them.

In your setup (radiating metal), you need some system where no entropy can leave (or enter) the system to use the second law of thermodynamics. This is "metal + all space where radiation from the metal is". In this system, entropy increases when the metal radiates and cools.

Ok . If i heat the metal by keeping it in contact with body at high temperature T2 then it gives heat q to the metal , So entropy of the body at T2 decreases, right??
That is correct. If q is the amount of energy being conducted from the body, then the body decreases in entropy by an amount q/T2.
Assuming that all energy is being transported between body and metal. The total entropy in both the metal and the body has to increase according to the Second Law. Therefore, the entropy in the metal as to increase by an amount equal to or greater than q/T2.
The second law has just been used to predict that the increase of entropy in the metal has to be equal to or greater than q/T2. However, the second law by itself doesn't provide the exact value for the increase in entropy of the metal.
The increase in entropy in the metal has two parts. The component of entropy that equals q/T2 is said to be transferred from the body to the metal. The part of the increase in metal entropy that exceeds q/T2 is said to have been created.

Last edited: